Method and device for generating a second image from a first image

ABSTRACT

Described are methods and devices for applying a color gamut mapping process on a first image to generate a second image, where the content of the first and second images is similar but the respective color spaces of the first and second images are different. The color gamut mapping process may be controlled using a color gamut mapping mode obtained from a bitstream where the color gamut mapping mode belongs to a set comprising at least two preset modes and an explicit parameters mode. If the obtained color gamut mapping mode is the explicit parameters mode and the color gamut mapping process is not enabled for the explicit parameters mode, the color gamut mapping process may be controlled by a substitute color gamut mapping mode determined from additional data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the national stage entry, under 35 U.S.C. § 371 ofInternational Application PCT/US2018/059351, filed Nov. 6, 2018, whichwas published in accordance with PCT Article 21(2) on May 16, 2019, inEnglish, and which claims the benefit of European Patent Application No.17306556.6, filed Nov. 9, 2017.

1. FIELD

The present principles generally relate to image/video processing fordistribution of High Dynamic Range (HDR) image. Particularly, but notexclusively, the technical field of the present principles are relatedto generating a second image from a first image by applying a colorgamut mapping process on the first image to generate the second imagewhich differs from the color space, the color gamut mapping processbeing controlled at least by a color gamut mapping mode obtained from abitstream.

2. BACKGROUND

The present section is intended to introduce the reader to variousaspects of art, which may be related to various aspects of the presentprinciples that are described and/or claimed below. This discussion isbelieved to be helpful in providing the reader with backgroundinformation to facilitate a better understanding of the various aspectsof the present principles. Accordingly, it should be understood thatthese statements are to be read in this light, and not as admissions ofprior art.

In the following, an image contains one or several arrays of samples(pixel values) in a specific image/video format which specifies allinformation relative to the pixel values of an image (or a video) andall information which may be used by a display and/or any other deviceto visualize and/or decode a image (or video) for example. An imagecomprises at least one component, in the shape of a first array ofsamples, usually a luma (or luminance) component, and, possibly, atleast one other component, in the shape of at least one other array ofsamples, usually a color component. Or, equivalently, the sameinformation may also be represented by a set of arrays of color samples,such as the traditional tri-chromatic RGB representation.

A pixel value is represented by a vector of C values, where C is thenumber of components. Each value of a vector is represented with anumber of bits which defines a maximal dynamic range of the pixelvalues.

Low-Dynamic-Range images (LDR images) are images whose luminance valuesare represented with a limited number of bits (most often 8 or 10). Thislimited representation does not allow correct rendering of small signalvariations, in particular in dark and bright luminance ranges. Inhigh-dynamic range images (HDR images), the signal representation isextended in order to maintain a high accuracy of the signal over itsentire range. In HDR images, pixel values representing luminance levelsare usually represented in floating-point format (either 32-bit or16-bit for each component, namely float or half-float), the most popularformat being openEXR half-float format (16-bit per RGB component, i.e.48 bits per pixel) or in integers with a long representation, typicallyat least 16 bits.

The arrival of the High Efficiency Video Coding (HEVC) standard (ITU-TH.265 Telecommunication standardization sector of ITU (October 2014),series H: audiovisual and multimedia systems, infrastructure ofaudiovisual services—coding of moving video, High efficiency videocoding, Recommendation ITU-T H.265) enables the deployment of new videoservices with enhanced viewing experience, such as Ultra HD broadcastservices. In addition to an increased spatial resolution, Ultra HD canbring a wider color gamut (WCG) and a higher dynamic range (HDR) thanthe Standard dynamic range (SDR) HD-TV currently deployed (with astandard color gamut SCG). Different solutions for the representationand coding of HDR/WCG video have been proposed (SMPTE 2014, “HighDynamic Range Electro-Optical Transfer Function of Mastering ReferenceDisplays, or SMPTE ST 2084, 2014, or Diaz, R., Blinstein, S. and Qu, S.“Integrating HEVC Video Compression with a High Dynamic Range VideoPipeline”, SMPTE Motion Imaging Journal, Vol. 125, Issue 1. February,2016, pp 14-21).

SDR backward compatibility with decoding and rendering devices is animportant feature in some video distribution systems, such asbroadcasting or multicasting systems.

Dual-layer coding is one solution to support this feature. However, dueto its multi-layer design, this solution is not adapted to alldistribution workflows.

An alternative is a single Layer HDR distribution solution as defined bythe ETSI recommendation ETSI TS 103 433-1 (August 2017). The reader mayalso refer to the IBC 2016 article (“A single-Layer HDR video codingframework with SDR compatibility”, E. Francois and L. Van de Kerkhof,IBC 2016) for more details. This single layer distribution solution isSDR compatible and leverages SDR distribution networks and servicesalready in place. It enables both high quality HDR rendering onHDR-enabled CE devices, while also offering high quality SDR renderingon SDR CE devices. This single layer distribution solution is based on asingle layer coding/decoding process and is codec independent (a 10 bitscodec is recommended). This single layer distribution solution uses sidemetadata (of a few bytes per video frame or scene) that can be used in apost-processing stage to reconstruct the HDR signal from a decoded SDRsignal and adapt the signal to the HDR-enabled CE devices. This singlelayer distribution solution preserves quality of HDR content (artisticintent), i.e. there is no visible impairment due to the SDRcompatibility feature in comparison with HEVC coding with a bit depth of8-bits to 10-bits per sample with 4:2:0 chroma sampling (so-called HEVCmain 10 profile).

To benefit of such quality, corresponding methods including pre/postprocessing and metadata have been proposed for various HDR bitstreams.Accordingly, the ETSI recommendation ETSI TS 103 433 is divided in 3parts SL-HDRx, each part addressing different distribution workflows.The ETSI recommendation ETSI TS 103 433-1 (2017-08), referred to asSL-HDR1 discloses a backward compatible system for transmitting an HDRvideo content based on a SDR video. The future ETSI recommendation ETSITS 103 433-2, referred to as SL-HDR2, is to be drafted for displayadaptation for PQ (Perceptual Quantization transfer function) video. Thefuture ETSI recommendation ETSI TS 103 433-3, referred to as SL-HDR3 isdrafted for a system for display adaptation for HLG (Hybrid Log-Gammatransfer function) video.

In particular, Annex D of ETSI TS 103 433-1 (SL-HDR1) provides thedescription of an invertible gamut mapping process that could apply whenthe input SDR picture of the SDR-to-HDR reconstruction process isprovided in a BT.709 color gamut (as specified by the variablesdrPicColourSpace), and is different from the target BT.2020 color gamutof the HDR picture (as specified by the variable hdrPicColourSpace). Inthis annex, color backward compatibility is defined such that the CEreceiver only supports BT.709 color space while the video to bedistributed using SL-HDR1 can support BT.2020 color space. Annex D ofthe second stable draft of ETSI TS 103 433-2 (SL-HDR2) also provides thedescription of a gamut mapping based on Annex D of SL-HDR1. The skilledin the art will notice that the invertible gamut mapping defined inAnnex D is not limited to SDR-to-HDR reconstruction but it compatiblewith any change of color spaces. In particular, the annex D of SL-HDR1discloses a set of modes for invertible gamut mapping that are signaledto the CE devices through indication of explicit parameters, of presetmethods or of an implementer dependent method. Low-end circuitry in somelow-cost CE devices may not be capable to process the explicitparameters mode. In this case, these devices would arbitrate thefollowing trade-off: either discard gamut mapping or use their own“blind” gamut mapping operator. In both cases, the experience of theuser is degraded. This is an issue when appointing SL-HDR technologiesas premium services delivery for high quality HDR rendering onHDR-enabled CE devices.

3. SUMMARY

The following presents a simplified summary of the present principles inorder to provide a basic understanding of some aspects of the presentprinciples. This summary is not an extensive overview of the presentprinciples. It is not intended to identify key or critical elements ofthe present principles. The following summary merely presents someaspects of the present principles in a simplified form as a prelude tothe more detailed description provided below.

The present principles set out to remedy at least one of the drawbacksof the prior art with a method for generating a second image from afirst image, the method comprising applying a color gamut mappingprocess on the first image to generate the second image, the content ofthe second image being similar to the content of the first image but thecolor space of the second image is different from the color space of thefirst image, the color gamut mapping process being controlled at leastby a color gamut mapping mode obtained from a bitstream and, the colorgamut mapping mode belonging to a set comprising at least two presetmodes and an explicit parameters mode. The method further comprises, incase the obtained color gamut mapping mode is the explicit parametersmode (gamutMappingMode is equal to 1) and in case the color gamutmapping process of the generating method is not enabled for the explicitparameters mode for generating the second image from the first image,determining a substitute color gamut mapping mode from additional data,the color gamut mapping process being then controlled by the substitutecolor gamut mapping mode instead of the explicit parameters mode.

According to an embodiment, determining color gamut mapping mode furthercomprises selecting a color gamut mapping mode among the at least twopreset modes.

According to an embodiment, additional data comprising at least one ofthe color space of the first image, the color space of the second image,the color space of the mastering display used to generate the firstimage, the color space of the mastering display used to generate thesecond image.

According to an embodiment, additional data are metadata signaled in thebitstream. According to an alternative embodiment, additional data aredetermined by the applicative environment, for instance additional data(such as the hdrDisplayColourSpace decribed below) is signaled or storedin a configuration file of the application.

According to an embodiment, the bitstream is an SL-HDR1 bitstream. Inthat case, determining substitute color gamut mapping mode comprisingselecting preset mode BT.709 to P3D65 gamut (substitute gamutMappingModeis set to “2”) when hdrDisplayColourSpace is equal to “2” and selectingpreset mode BT.709 to BT.2020 gamut (substitute gamutMappingMode is setto “3”) when hdrDisplayColourSpace is equal to “1”.

According to an embodiment, the bitstream is an SL-HDR2 bitstream. Inthat case, determining substitute color gamut mapping mode comprisingselecting preset mode P3D65 to BT.709 gamut (substitute gamutMappingModeis set to “4”) when hdrDisplayColourSpace is equal to “2” and selectingpreset mode BT.2020 to BT.709 gamut (substitute gamutMappingMode is setto “5”) when hdrDisplayColourSpace is equal to “1”.

According to an embodiment, the color gamut mapping process isdetermined to not be capable of using the explicit parameters modeaccording to an indicator. In a variant the indicator stored in thedevice implementing the method.

According to other of their aspects, the present principles relate to adevice for generating a second image from a first image, a computerprogram product comprising program code instructions to execute thesteps of the disclosed method when this program is executed on acomputer, a processor readable medium having stored therein instructionsfor causing a processor to perform at least the steps of the disclosedmethod when this program is executed on a computer. Any of theembodiments described for the method is compatible with the device.

4. BRIEF DESCRIPTION OF DRAWINGS

In the drawings, examples of the present principles are illustrated. Itshows:

FIG. 1 shows an end-to-end workflow supporting content production anddelivery to HDR and SDR displays;

FIG. 2 shows non-limitative examples in accordance with single-layerdistribution bitstream SL-HDRx of the end-to-end workflow of FIG.1;

FIG. 3 a depicts in more details the pre-processing stage with SL-HDRxbitstream;

FIG. 3 b depicts in more details the post-processing stage with SL-HDR1bitstream;

FIG. 3 c depicts in more details the post-processing stage with SL-HDR2or with SL-HDR3 bitstream;

FIG. 4 shows a block diagram of the steps of a method for generating asecond image from a first image in accordance with examples of thepresent principles;

FIG. 5 a illustrates a non-limitative example in accordance with SL-HDRbitstream of a block diagram of the steps of a method for generating asecond image from a first image;

FIG. 5 b illustrates a non-limitative example in accordance with SL-HDR1bitstream of a block diagram of the step of determining a substitutegamut mapping mode;

FIG. 5 c illustrates a non-limitative example in accordance with SL-HDR2bitstream of a block diagram of the step of determining a substitutegamut mapping mode;

FIG. 6 shows an example of an architecture of a device in accordancewith an example of present principles; and

FIG. 7 shows two remote devices communicating over a communicationnetwork in accordance with an example of present principles;

Similar or same elements are referenced with the same reference numbers.

5. DESCRIPTION OF EXAMPLE OF THE PRESENT PRINCIPLES

The present principles will be described more fully hereinafter withreference to the accompanying figures, in which examples of the presentprinciples are shown. The present principles may, however, be embodiedin many alternate forms and should not be construed as limited to theexamples set forth herein. Accordingly, while the present principles aresusceptible to various modifications and alternative forms, specificexamples thereof are shown by way of examples in the drawings and willherein be described in detail. It should be understood, however, thatthere is no intent to limit the present principles to the particularforms disclosed, but on the contrary, the disclosure is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the present principles as defined by the claims.

The terminology used herein is for the purpose of describing particularexamples only and is not intended to be limiting of the presentprinciples. As used herein, the singular forms “a”, “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises”, “comprising,” “includes” and/or “including” when used inthis specification, specify the presence of stated features, integers,steps, operations, elements, and/or components but do not preclude thepresence or addition of one or more other features, integers, steps,operations, elements, components, and/or groups thereof. Moreover, whenan element is referred to as being “responsive” or “connected” toanother element, it can be directly responsive or connected to the otherelement, or intervening elements may be present. In contrast, when anelement is referred to as being “directly responsive” or “directlyconnected” to other element, there are no intervening elements present.As used herein the term “and/or” includes any and all combinations ofone or more of the associated listed items and may be abbreviated as“/”.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement without departing from the teachings of the present principles.

Although some of the diagrams include arrows on communication paths toshow a primary direction of communication, it is to be understood thatcommunication may occur in the opposite direction to the depictedarrows.

Some examples are described with regard to block diagrams andoperational flowcharts in which each block represents a circuit element,module, or portion of code which comprises one or more executableinstructions for implementing the specified logical function(s). Itshould also be noted that in other implementations, the function(s)noted in the blocks may occur out of the order noted. For example, twoblocks shown in succession may, in fact, be executed substantiallyconcurrently or the blocks may sometimes be executed in the reverseorder, depending on the functionality involved.

Reference herein to “in accordance with an example” or “in an example”means that a particular feature, structure, or characteristic describedin connection with the example can be included in at least oneimplementation of the present principles. The appearances of the phrasein accordance with an example” or “in an example” in various places inthe specification are not necessarily all referring to the same example,nor are separate or alternative examples necessarily mutually exclusiveof other examples.

Reference numerals appearing in the claims are by way of illustrationonly and shall have no limiting effect on the scope of the claims.

While not explicitly described, the present examples and variants may beemployed in any combination or sub-combination.

The present principles are described for coding/decoding/reconstructingan image but extends to the coding/decoding/reconstruction of a sequenceof images (video) because each image of the sequence is sequentiallyencoded/decoded/reconstructed as described below.

FIG. 1 shows an end-to-end workflow supporting content production anddelivery to HDR and SDR displays. It involves a single-layer SDR/HDRencoding-decoding with side metadata as defined, for example, in ETSI TS103 433.

At a pre-processing stage, an incoming HDR video is decomposed in an SDRvideo and metadata. The SDR video is then encoded with any SDR videocodec and an SDR bitstream is carried throughout an existing legacy SDRdistribution network with accompanying metadata conveyed on a specificchannel or embedded in the SDR bitstream. Preferably, the video coded isan HEVC codec such as the H.265/HEVC codec or H.264/AVC. The metadataare typically carried by SEI messages when used in conjunction with anH.265/HEVC or H.264/AVC codec.

The SDR bitstream is decoded and a decoded SDR video is then availablefor an SDR Consumer Electronics (CE) display.

Next, at a post-processing stage, which is functionally the inverse ofthe pre-processing stage, the HDR video is reconstructed from thedecoded SDR video and from metadata obtained from a specific channel orfrom the SDR bitstream.

FIG. 2 shows non-limitative examples in accordance with single-layerdistribution bitstream SL-HDRx of the end-to-end workflow of FIG. 1 .

The SL-HDR1 (ETSI TS 103 433-1) specification includes dynamic metadata(CC1+TM metadata) for signal reconstruction (metadata may be embedded ina user-defined SEI message for HEVC and include the mastering displaycolor volume/space metadata and gamut mapping metadata to be used inpost-processing); SDR-to-HDR reconstruction process specifying thepost-processing to reconstruct the HDR signal from the decoded SDRsignal and SL-HDR1 metadata. SL-HDR1 also describes the pre-processingto generate an SDR signal from the HDR content; the invertible gamutmapping process; the HDR-to-HDR display adaptation process; anerror-concealment strategy in case metadata are lost or corrupted;signaling in CTA-861-G (notably for usage in HDMI2.1).

The extension SL-HDR2 (ETSI TS 103 433-2 in a second stable draft stageat the time of writing) addresses display adaptation for PQ10 and theextension SL-HDR3 (ETSI TS 103 433-3 in an early draft stage at the timeof writing) addresses display adaptation and hue shift correction forHLG10. These extensions are built upon SL-HDR1 specification and reusethe same metadata and the same pixel-loop hardware in thepost-processing. However, the post-processing is driven using adifferent software layer implementation.

Accordingly, an SL-HDRx bitstream is a bitstream containing a codedvideo and associated SL-HDRx metadata (as specified in Annex A or B ofTS 103 433-X)

FIG. 3 a depicts in more details the pre-processing stage according tothe non-limiting example of SL-HDR1 for the sake of clarity. However,the present principles are compatible with SL-HDR2 or SL-HDR3. It shouldbe noted that the “color gamut mapping” term also cover “inverse colorgamut mapping”. Indeed, at the post-processor side, in SL-HDR1 aninverse gamut mapping process (extension of gamut) is applied while inSL-HDR2 a gamut mapping process (compression of gamut) is applied. Atthe pre-proc, SL-HDR1 and SL-HDR2/3 decomposes the signal fromHDR-to-SDR. SL-HDR1 distributes the obtained SDR video and metadatawhile SL-HDR2 distributes an HDR10 version of the HDR video (they may bethe same as masters are often HDR10) and metadata.

The core component of the pre-processing stage is the HDR-to-SDRdecomposition that generates an SDR video and dynamic metadata from theHDR video. More precisely, the HDR-to-SDR decomposition aims atconverting a HDR video represented in a specific input format (here RGB)to a SDR video represented in a specific output format (here Y_(SDR),U_(SDR),V_(SDR)) but the present principles are not limited to specificinput/output format (color space or gamut).

Optionally, the format of the HDR video, respectively the format of theSDR video, may be adapted to said specific input format, respectivelyspecific output format. Accordingly, the content of the HDR video issimilar to the content of the SDR video, but the color space of the SDRvideo is different from the color space of the HDR video. Saidinput/output format adaptation may include color space conversion and/orcolor gamut mapping. Usual format adapting processes may be used such asRGB-to-YUV or YUV-to-RGB conversion, BT.709-to-BT.2020 orBT.2020-to-BT.709, down-sampling or up-sampling chroma components, etc.Note that the well-known YUV color space refers also to the well-knownYCbCr or Y′CbCr or Y′C′bC′r in the prior art.

In particular, the annex D of SL-HDR1 specification describes aninvertible gamut mapping driven by parameters/metadata carried in thebitstream so that the reconstructed extended HDR WCG signal quality iscontrolled and approved by operators or content creators thanks to apossible Quality Check stage during the generation phase of theparameters (before distribution). This controlled gamut mapping methodadvantageously enhances content fidelity over “blind” (i.e. not guidedby metadata or parameters approved during the Quality Check stage) gamutmapping methods that ignore characteristics of the original contentdecomposed in a SDR SCG signal before transmission. The invertible gamutmapping specified in Annex D of ETSI SL-HDR1 specification coversdifferent gamut mapping modes: explicit parameters carriage (wherein afull set of parameter values used in the gamut mapping process is sentto the CE devices), presets (wherein a set of predetermined parametervalues are referred to by an index) or implementer dependent method(corresponding to blind methods).

TABLE 1 Gamut mapping mode present in SL-HDR1 Value of gamutMappingModeGamut mapping mode 0 Implementation dependent method 1 Explicitparameters (see clause 6.3.9) 2 Preset #1: BT.709 to P3D65 gamut (seeTable 12) 3 Preset #2: BT.709 to BT.2020 gamut (see Table 13) 4-63Reserved for future use 64-127 Unspecified 128-255  Reserved for futureuse

FIG. 3 b depicts in more details the post-processing stage according tothe non-limiting example of SL-HDR1 for the sake of clarity. However,the presents principles are compatible with SL-HDR2 or SL-HDR3, as shownon FIG. 3 c , using respectively the wording first video and secondvideo instead of SDR video and HDR video, wherein the first videocorrespond to the encoded/decoded video of the bitstream (being SDR,HDR10 or HLG), and wherein the second video (HDR/SDR) corresponds to therendered video on CE devices (Display) after post-processing.

The core component of the post-processing stage is the HDRreconstruction that generates a reconstructed HDR video from a decodedSDR video and metadata. More precisely, the HDR reconstruction aims atconverting SDR video represented in a specific input format (hereY_(SDR),U_(SDR),V_(SDR)) to an output HDR video represented in aspecific output format (here RGB) but the present principles are notlimited to specific input/output specific formats (color space or colorgamut or color primaries).

Optionally, the format of the reconstructed HDR video may be adapted totargeted system characteristics (e.g. a Set-Top-Box, a connected TVnoted Display) and/or an inverse gamut mapping may be used when thedecoded SDR video (input of the HDR reconstruction stage) and thereconstructed HDR video (output of the HDR reconstruction stage) arerepresented in different color spaces and/or gamut. Input or outputformat adapting may include color space conversion and/or color gamutmapping. Usual format adapting processes may be used such as RGB-to-YUV(or YUV-to-RGB) conversion, BT.709-to-BT.2020 or BT.2020-to-BT.709,down-sampling or up-sampling chroma components, etc. Annex E of the ETSIspecification SL-HDR1 provides an example of format adapting processesand Annex D of the ETSI specification SL-HDR1 provides inverse gamutmapping processes. The HDR reconstruction is the functional inverse ofthe HDR-to-SDR decomposition.

Thus, as previously exposed for pre-processing, in a particular example,the gamut mapping GM (or inverse gamut mapping IGM depending of thecolor spaces of the input/output image) is advantageously controlled bya gamut mapping mode obtained from the bitstream and comprising anexplicit parameters mode, at least two presets (corresponding tostandardized color space conversion). However, some low-cost CE devicesmay not be enabled for processing the explicit parameters mode. In thiscase, these devices either discard gamut mapping (presenting areconstructed content with a standard color palette) or use their own“blind” gamut mapping operator (ignoring the original extended/widecolor gamut of the original content). A method for post processing thevideo from including a non-blind gamut mapping GM that accounts for theinput extended/wide color gamut of the original content is thereforedesirable.

FIG. 3 c depicts in more details the post-processing stage according tothe SL-HDR2 or SL-HDR3. for SL-HDR2. At the post-proc, the gamut mappingis applied on reconstructed SDR or HDR with lower peak luminance thanthe HDR signal carried on the distribution networks, and the gamutmapping controlled by metadata.

The core component of the post-processing stage is the HDR/SDRreconstruction that generates a reconstructed HDR video or reconstructedSDR video from the decoded HDR video and metadata. More precisely, theHDR reconstruction aims at converting HDR video represented in aspecific input format to an output HDR/SDR video represented in aspecific output format but the present principles are not limited tospecific input/output specific formats (color space or color gamut orcolor primaries).

Optionally, the format of the reconstructed HDR/SDR video may be adaptedto display characteristics (e.g. a Set-Top-Box, a connected TV) and/oran inverse gamut mapping may be used when the decoded HDR video (inputof the HDR/SDR reconstruction stage) and the reconstructed HDR/SDR video(output of the HDR reconstruction stage) are represented in differentcolor spaces and/or gamut. Input or output format adapting may includecolor space conversion and/or color gamut mapping. Usual format adaptingprocesses may be used such as RGB-to-YUV (or YUV-to-RGB) conversion,BT.709-to-BT.2020 or BT.2020-to-BT.709, down-sampling or up-samplingchroma components, etc.

Thus, as previously exposed for pre-processing, in a particular example,the gamut mapping GM (or inverse gamut mapping IGM depending of thecolor spaces of the input/output image) is advantageously controlled bya gamut mapping mode obtained from the bitstream and comprising anexplicit parameters mode, at least two presets (corresponding tostandardized color space conversion). However, some low-cost CE devicesmay not be enabled for processing the explicit parameters mode. In thiscase, these devices either discard gamut mapping (presenting areconstructed content with a standard color palette) or use their own“blind” gamut mapping operator (ignoring the original extended/widecolor gamut of the original content). Again, a method for postprocessing the video from including a non-blind gamut mapping GM thataccounts for the input extended/wide color gamut of the original contentis therefore desirable.

FIG. 4 shows a block diagram of the steps of a method for generating asecond image from a first image in accordance with examples of thepresent principles.

The method comprises, in a step 47, applying a color gamut mappingprocess (or an inverse gamut mapping process) on the first image togenerate the second image. According to examples described above, thecontent of the second image is similar to the content of the firstimage, but the color space of the second image is different from thecolor space of the first image. For illustrative purpose, the secondimage may be an WCG image reconstructed in a BT 2020 color space and thefirst image may be a SCG image of a BT 709 color space (SL-HDR1 case);or the first image may be an SCG image of a BT 709 color space and thesecond image may be a WCG image in a BT 2020 color space (SL-HDR2 orSL-HDR3 case). According to a particular feature, the color gamutmapping process is controlled at least by a color gamut mapping mode,gamutMappingMode, obtained from a bitstream. Such color gamut mappingmode informs the CE device of the color gamut mapping used atpre-processing stage that would ensure high fidelity in color renderingafter the post-processing stage between the master video content and thevideo content displayed by the CE device. According to anotherparticular feature, the color gamut mapping mode belongs to a setcomprising at least two preset modes and an explicit parameters mode asexposed for SL-HDRx specifications wherein the explicit parameters modeoffers full parameters control of the gamut mapping process GM/IGM.

To that end, in step 41, the gamutMappingMode is obtained from thebitstream. The gamutMappingMode may be carried by metadata and may beobtained, for example, by parsing SEI messages as explained above. In astep 42, the gamutMappingMode is tested to check whether it is set toexplicit parameters mode. In case the gamutMappingMode is not theexplicit parameters mode, for instance if gamutMappingMode is set to apreset mode, the gamutMappingMode is passed to the step 47 forcontrolling the gamut mapping (GM/IGM). For example, presets are definedfor different (possibly ranges of) content/image format color primaries.

In case the gamutMappingMode is the explicit parameters mode, in a step43, the gamut mapping process is tested to check whether it is enabledfor the explicit parameters mode, i.e. if the gamut mapping process iscapable of applying a programmable gamut mapping that accounts with thegamut mapping parameters signaled with the explicit parameters mode.According to an embodiment of the method, an indicator 44 isrepresentative of the explicit GM/IGM capability and is used todetermine if the gamut mapping process is enabled for the explicitparameters mode. Such indicator is stored in a register. In case thegamut mapping process is enabled for the explicit parameters mode, thegamutMappingMode, along with the gamut mapping parameters, is passed tothe step 47 for controlling the gamut mapping process (GM/IGM). In casethe gamutMappingMode is the explicit parameters mode and in case thecolor gamut mapping process of the method is not enabled for thegamutMappingMode, in a step 45 a substitute color gamut mapping mode,substitute gamutMappingMode, is determined from additional data 46, thenthe substitute gamutMappingMode is passed to the step 47 for controllingthe gamut mapping process (GM/IGM).

According to an embodiment, a switch 48 controlled by the outputs oftests 42 and 43 connects the gamutMappingMode or the substitutegamutMappingMode to the gamut mapping process 47.

According to an embodiment of the step 45, determining color gamutmapping mode further comprises selecting a color gamut mapping modeamong the at least two preset modes. Advantageously, such embodimentpreserves most of the benefit of an explicit gamut mode for processinggamut mapping within the presets-only-capable receiver/devices since thepreset selection is not blind but controlled by additional data.

According to further embodiment of the step 45, the additional datacomprises at least one of the color space of the first image, the colorspace of the second image, the color space of the mastering display usedto generate the first image, the color space of the mastering displayused to generate the second image, the color space of the targetdisplay, the color space of an input picture of SL-HDR pre-processing.In other words, the mapping toward a preset or the other(s) aredetermined for instance by the knowledge of the original WCG contentcolor space or of the color space of the mastering display used to gradethe original WCG content or the color space supported by the targeteddisplay.

FIG. 5 a illustrates a non-limitative example in accordance with SL-HDRbitstream of a block diagram of the steps of a method for generating asecond image from a first image.

This method is based on any HDR or SDR reconstruction process requiringa SDR/HDR10/HLG image and metadata as input.

It is expected, during first deployment of SL-HDR, that allSL-HDR-capable (i.e. receiver that can process SL-HDR metadata in anembedded SL-HDR post-processor module) receiver supports a simplifiedversion, for instance based on a limited set of 3D LUT, of the gamutmapping process while not being enabled for analytical process such asdefined in Annex D. Accordingly, a receiver device that is not capableto process or interpret these explicit parameters may indicate thiscapability thanks to a device internal register, or thanks to a bit inVSVDB or HDMI HDR Dynamic Metadata Data Block or EDID in the receiverprocessing the gamut mapping.

As described for FIG. 4 , the receiver parses SEI metadata from theSL-HDR bitstream to obtain at least the gamutMappingMode and one of thehdrDisplayColourSpace or the hdrMasterDisplayColourSpace (See Annex A ofSL-HDR1). If the gamutMappingMode is set to ‘1’, meaning as shown intable 1 that the gamut mapping mode is the explicit parameter mode, andif the SL-HDR-capable receiver is only enabled for preset modes, then amapping module determines a substitute gamutMappingMode based on one ofhdrDisplayColourSpace or the hdrMasterDisplayColourSpace.

FIG. 5 b and FIG. 5 c respectively illustrate a non-limitative examplein accordance with SL-HDR1 bitstream and SL-HDR2 bitstream of themapping module.

In the case of SL-HDR1, the first image is an image with standard colorgamut (such as BT.709), the second image is an image with WCG (such asP3D65, BT.2020) and the mapping module selects the preset mode BT.709 toP3D65 gamut (gamutMappingMode is set to “2”) when hdrDisplayColourSpace(or hdrMasterDisplayColourSpace) is equal to “2” and selects the presetmode BT.709 to BT.2020 gamut (gamutMappingMode is set to “3”) whenhdrDisplayColourSpace (or hdrMasterDisplayColourSpace) is equal to “1”.

In the case of SL-HDR2, the first image is an image with WCG (such as toP3D65, BT.2020), the second image is an image with standard color gamut(such as BT.709) and the mapping module selects the preset mode P3D65 toBT.709 gamut (gamutMappingMode is set to “4”) when hdrDisplayColourSpace(or hdrMasterDisplayColourSpace) is equal to “2” and selects the presetmode BT.2020 to BT.709 gamut (gamutMappingMode is set to “5”) whenhdrDisplayColourSpace (or hdrMasterDisplayColourSpace) is equal to “1”.

It is noted that this mapping module can apply to every receiver/deviceimplementing either SL-HDR1, 2 or 3 specifications.

According to yet other embodiments, instead of usinghdrDisplayColourSpace, one can use src_mdcv_primaries_*syntax elementsspecified in Annex A.2.2.2 of SL-HDR1 specification, ordisplay_primaries_*syntax elements specified in HEVC specification, orservice signaling information from ATSC3.0 or CN-HDR at the transportlayer. Typically, mapping to presets can be inspired from Table A.5 fordisplay_primaries_*syntax elements or Table A.4 forsrc_mdcv_primaries_*elements, the lost or corrupted parameters arerecovered from at least one set of pre-determined parameter valuespreviously stored.

On FIG. 1-5 c, the modules are functional units, which may or not be inrelation with distinguishable physical units. For example, these modulesor some of them may be brought together in a unique component orcircuit, or contribute to functionalities of a software. A contrario,some modules may potentially be composed of separate physical entities.The apparatus which are compatible with the present principles areimplemented using either pure hardware, for example using dedicatedhardware such ASIC or FPGA or VLSI, respectively «Application SpecificIntegrated Circuit», «Field-Programmable Gate Array», «Very Large ScaleIntegration», or from several integrated electronic components embeddedin a device or from a blend of hardware and software components.

FIG. 6 represents an exemplary architecture of a device 60 which may beconfigured to implement a method described in relation with FIG. 1-5 c.

Device 60 comprises following elements that are linked together by adata and address bus 61:

-   a microprocessor 62 (or CPU), which is, for example, a DSP (or    Digital Signal Processor);-   a ROM (or Read Only Memory) 63;-   a RAM (or Random Access Memory) 64;-   an I/O interface 65 for reception of data to transmit, from an    application; and-   a battery 66

In accordance with an example, the battery 66 is external to the device.In each of mentioned memory, the word «register» used in thespecification can correspond to area of small capacity (some bits) or tovery large area (e.g. a whole program or large amount of received ordecoded data). The ROM 63 comprises at least a program and parameters.The ROM 63 may store algorithms and instructions to perform techniquesin accordance with present principles. When switched on, the CPU 62uploads the program in the RAM and executes the correspondinginstructions.

RAM 64 comprises, in a register, the program executed by the CPU 62 anduploaded after switch on of the device 60, input data in a register,intermediate data in different states of the method in a register, andother variables used for the execution of the method in a register.

The implementations described herein may be implemented in, for example,a method or a process, an apparatus, a software program, a data stream,or a signal. Even if only discussed in the context of a single form ofimplementation (for example, discussed only as a method or a device),the implementation of features discussed may also be implemented inother forms (for example a program). An apparatus may be implemented in,for example, appropriate hardware, software, and firmware. The methodsmay be implemented in, for example, an apparatus such as, for example, aprocessor, which refers to processing devices in general, including, forexample, a computer, a microprocessor, an integrated circuit, or aprogrammable logic device. Processors also include communicationdevices, such as, for example, computers, cell phones, portable/personaldigital assistants (“PDAs”), and other devices that facilitatecommunication of information between end-users.

In accordance with an example of pre-processing and/or encoding device,the video or an image of a video is obtained from a source. For example,the source belongs to a set comprising:

-   -   a local memory (63 or 64), e.g. a video memory or a RAM (or        Random Access Memory), a flash memory, a ROM (or Read Only        Memory), a hard disk;    -   a storage interface (65), e.g. an interface with a mass storage,        a RAM, a flash memory, a ROM, an optical disc or a magnetic        support;    -   a communication interface (65), e.g. a wireline interface (for        example a bus interface, a wide area network interface, a local        area network interface) or a wireless interface (such as a IEEE        802.11 interface or a Bluetooth® interface); and    -   an image capturing circuit (e.g. a sensor such as, for example,        a CCD (or Charge-Coupled Device) or CMOS (or Complementary        Metal-Oxide-Semiconductor)).

In accordance with an example of the post-processing and/or decodingdevice, the decoded video or reconstructed video is sent to adestination; specifically, the destination belongs to a set comprising:

-   -   a local memory (63 or 64), e.g. a video memory or a RAM, a flash        memory, a hard disk;    -   a storage interface (65), e.g. an interface with a mass storage,        a RAM, a flash memory, a ROM, an optical disc or a magnetic        support;    -   a communication interface (65), e.g. a wireline interface (for        example a bus interface (e.g. USB (or Universal Serial Bus)), a        wide area network interface, a local area network interface, a        HDMI (High Definition Multimedia Interface) interface) or a        wireless interface (such as a IEEE 802.11 interface, WiFi® or a        Bluetooth® interface); and    -   a display.

In accordance with examples of pre-processing and/or encoding, thebitstream and/or the other bitstream carrying the metadata are sent to adestination. As an example, one of these bitstream or both are stored ina local or remote memory, e.g. a video memory (64) or a RAM (64), a harddisk (63). In a variant, one or both of these bitstreams are sent to astorage interface (65), e.g. an interface with a mass storage, a flashmemory, ROM, an optical disc or a magnetic support and/or transmittedover a communication interface (65), e.g. an interface to a point topoint link, a communication bus, a point to multipoint link or abroadcast network.

In accordance with examples of post-processing and/or decoding, thebitstream and/or the other bitstream carrying the metadata is obtainedfrom a source. Exemplarily, the bitstream is read from a local memory,e.g. a video memory (64), a RAM (64), a ROM (63), a flash memory (63) ora hard disk (63). In a variant, the bitstream is received from a storageinterface (65), e.g. an interface with a mass storage, a RAM, a ROM, aflash memory, an optical disc or a magnetic support and/or received froma communication interface (65), e.g. an interface to a point to pointlink, a bus, a point to multipoint link or a broadcast network.

In accordance with examples, device 60 being configured to implement apre-processing method and/or encoding method as described above, belongsto a set comprising:

-   -   a mobile device;    -   a communication device;    -   a game device;    -   a tablet (or tablet computer);    -   a laptop;    -   a still image camera;    -   a video camera;    -   an encoding chip;    -   a still image server; and    -   a video server (e.g. a broadcast server, a video-on-demand        server or a web server).

In accordance with examples, device 60 being configured to implement apost-processing method and or decoding method as described above,belongs to a set comprising:

-   -   a mobile device;    -   a communication device;    -   a game device;    -   a set top box;    -   a TV set;    -   a tablet (or tablet computer);    -   a laptop;    -   a display and    -   a decoding chip.

According to an example of the present principles, illustrated in FIG. 7, in a transmission context between two remote devices A and B over acommunication network NET, the device A comprises a processor inrelation with memory RAM and ROM which are configured to implement amethod for encoding an image as described above and the device Bcomprises a processor in relation with memory RAM and ROM which areconfigured to implement a method for decoding as described above.

In accordance with an example, the network is a broadcast network,adapted to broadcast still images or video images from device A todecoding devices including the device B.

A signal, intended to be transmitted by the device A, carries thebitstream and/or the other bitstream carrying the metadata. Thebitstream comprises an encoded video as explained before. This signalfurther comprises metadata relative to parameter values used forreconstructing a video from said decoded video.

The signal further comprises an indicator identifying a set ofpre-determined parameters used for recovering lost or corruptedparameters.

According to an embodiment, said indicator is hidden in metadata carriedby the signal.

Implementations of the various processes and features described hereinmay be embodied in a variety of different equipment or applications.Examples of such equipment include an encoder, a decoder, apost-processor processing output from a decoder, a pre-processorproviding input to an encoder, a video coder, a video decoder, a videocodec, a web server, a set-top box, a laptop, a personal computer, acell phone, a PDA, and any other device for processing a image or avideo or other communication devices. As should be clear, the equipmentmay be mobile and even installed in a mobile vehicle.

Additionally, the methods may be implemented by instructions beingperformed by a processor, and such instructions (and/or data valuesproduced by an implementation) may be stored on a computer readablestorage medium. A computer readable storage medium can take the form ofa computer readable program product embodied in one or more computerreadable medium(s) and having computer readable program code embodiedthereon that is executable by a computer. A computer readable storagemedium as used herein is considered a non-transitory storage mediumgiven the inherent capability to store the information therein as wellas the inherent capability to provide retrieval of the informationtherefrom. A computer readable storage medium can be, for example, butis not limited to, an electronic, magnetic, optical, electromagnetic,infrared, or semiconductor system, apparatus, or device, or any suitablecombination of the foregoing. It is to be appreciated that thefollowing, while providing more specific examples of computer readablestorage mediums to which the present principles can be applied, ismerely an illustrative and not exhaustive listing as is readilyappreciated by one of ordinary skill in the art: a portable computerdiskette; a hard disk; a read-only memory (ROM); an erasableprogrammable read-only memory (EPROM or Flash memory); a portablecompact disc read-only memory (CD-ROM); an optical storage device; amagnetic storage device; or any suitable combination of the foregoing.

The instructions may form an application program tangibly embodied on aprocessor-readable medium.

Instructions may be, for example, in hardware, firmware, software, or acombination. Instructions may be found in, for example, an operatingsystem, a separate application, or a combination of the two. A processormay be characterized, therefore, as, for example, both a deviceconfigured to carry out a process and a device that includes aprocessor-readable medium (such as a storage device) having instructionsfor carrying out a process. Further, a processor-readable medium maystore, in addition to or in lieu of instructions, data values producedby an implementation.

As will be evident to one of skill in the art, implementations mayproduce a variety of signals formatted to carry information that may be,for example, stored or transmitted. The information may include, forexample, instructions for performing a method, or data produced by oneof the described implementations. For example, a signal may be formattedto carry as data the rules for writing or reading the syntax of adescribed example of the present principles, or to carry as data theactual syntax-values written by a described example of the presentprinciples. Such a signal may be formatted, for example, as anelectromagnetic wave (for example, using a radio frequency portion ofspectrum) or as a baseband signal. The formatting may include, forexample, encoding a data stream and modulating a carrier with theencoded data stream. The information that the signal carries may be, forexample, analog or digital information. The signal may be transmittedover a variety of different wired or wireless links, as is known. Thesignal may be stored on a processor-readable medium.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. For example,elements of different implementations may be combined, supplemented,modified, or removed to produce other implementations. Additionally, oneof ordinary skill will understand that other structures and processesmay be substituted for those disclosed and the resulting implementationswill perform at least substantially the same function(s), in at leastsubstantially the same way(s), to achieve at least substantially thesame result(s) as the implementations disclosed. Accordingly, these andother implementations are contemplated by this application.

The invention claimed is:
 1. A method for generating a second image froma first image, the method comprising: applying a color gamut mappingprocess on the first image to generate the second image, a content ofthe second image corresponding to a content of the first image butwherein a color space of the second image is different from a colorspace of the first image, the color gamut mapping process beingcontrolled at least by a color gamut mapping mode obtained from abitstream and, the color gamut mapping mode belonging to a setcomprising at least two preset modes and an explicit parameters mode;the method further comprising, in case the obtained color gamut mappingmode is the explicit parameters mode and in case the color gamut mappingprocess of the method is not enabled for the explicit parameters modefor generating the second image from the first image, determining asubstitute color gamut mapping mode from additional data, the colorgamut mapping process being controlled by the substitute color gamutmapping mode.
 2. The method of claim 1, wherein determining color gamutmapping mode further comprises selecting a color gamut mapping modeamong the at least two preset modes.
 3. The method of claim 1, whereinthe additional data comprises at least one of the color space of thefirst image, the color space of the second image, a color space of themastering display used to generate the first image, a color space of themastering display used to generate the second image, a color space ofthe target display, or a color space of an input picture of SL-HDRpre-processing.
 4. The method of claim 3, wherein the additional data ismetadata signaled in the bitstream.
 5. The method of claim 4, whereinthe bitstream is SL-HDR1 bitstream.
 6. The method of claim 5, whereindetermining the substitute color gamut mapping mode comprises selectingpreset mode BT.709 to P3D65 gamut by setting substitute gamutMappingModeto “2” when hdrDisplayColourSpace is equal to “2”, and selecting presetmode BT.709 to BT.2020 gamut by setting substitute gamutMappingMode to“3” when hdrDisplayColourSpace is equal to “1”.
 7. The method of claim4, wherein the bitstream is SL-HDR2 bitstream.
 8. The method of claim 7,wherein determining the substitute color gamut mapping mode comprisesselecting preset mode P3D65 to BT.709 gamut by setting substitutegamutMappingMode to “4” when hdrDisplayColourSpace is equal to “2”, andselecting preset mode BT.2020 to BT.709 gamut by setting substitutegamutMappingMode to “5” when hdrDisplayColourSpace is equal to “1”. 9.The method of claim 1, further comprising determining whether the colorgamut mapping process is not capable of using the explicit parametersmode according to an indicator.
 10. A non-transitory processor readablemedium having stored therein instructions for causing a processor toperform at least the steps of the method according to claim 1 whenexecuted on a computer.
 11. A device for generating a second image froma first image, the device comprising: one or more processors, and atleast one memory, wherein the one or more processors is configured to:color gamut map the first image to generate the second image, a contentof the second image corresponding to a content of the first image butwherein a color space of the second image is different from a colorspace of the first image, the color gamut mapping being controlled by acolor gamut mapping mode obtained from a bitstream, and the color gamutmapping mode belonging to a set comprising at least two preset modes andan explicit parameters mode; and wherein the one or more processors isconfigured to determine a substitute color gamut mapping mode fromadditional data in case the obtained color gamut mapping mode is theexplicit parameters mode and in case the processor is not enabled forgamut mapping with the explicit parameters mode, the color gamut mappingbeing controlled by the substitute color gamut mapping mode.
 12. Thedevice of claim 11, wherein the one or more processors are configured todetermine whether the gamut mapping is not enabled for the explicitparameters mode according to an indicator stored in the device.
 13. Thedevice of claim 11, wherein the device is a mobile device; acommunication device; a game device; a set top box; a TV set; a Blu-Raydisc player; a player; a tablet; a laptop; a display; a camera; or adecoding chip.